A program to investigate the structure and properties of both DNA and RNA tumor viruses is proposed. A major component will be the application of recombinant DNA methods to viral systems. For adenovirus these methods can be used to generate new types of mutants with which to analyze viral physiology. For a variety of retroviruses, including the Friend, Abelson, Moloney, AKR and MCF leukemia viruses as well as the Moloney sarcoma virus, this approach offers the opportunity to learn about the architectural details of their genomes with great precision. It should also allow the application to retroviruses of genetic methods that have previously only been applicable to DNA tumor viruses. Among the proposed areas of investigation are studies of the details of the mechanism of reverse transcription, the transfectability of transformed phenotypes of chemically transformed cells, the genetic organization and expression of the spleen focus forming virus component of the Friend leukemia virus complex, the genetic structure of endogenous retroviruses of wild mice and the expresson of adenovirus genes.